Parts: 4
Writer: Peter Grimwade
Airdates: 1 February 1983 - 9 February 1983
Production Code: 6F

Doctor: fifth
Companions: Nyssa, Tegan, Turlough

Megalomaniac of the Week: Black Guardian
Special Guest: the Brigadier

Story: At a private boys' school in England, 1983, a mischievous young lad named Turlough persuades his friend Ibbotson to take an antique car out for a spin - but Turlough's gleefully reckless driving results in a serious accident for both them and the car. Turlough finds himself in a strange realm looking down on his supine body from above, and soon he is joined by the figure of the Black Guardian. The Guardian wishes to make a deal with Turlough - he can depart the Earth, a planet he hates, on the condition that he kills a being of the most supreme evil...the Doctor. Desperate to escape, Turlough agrees.

Meanwhile, in the TARDIS, the Doctor, Nyssa and Tegan narrowly avoid a collision with a mysterious ship when the Doctor manages to materialize on board. The ship is plush but abandoned, and Nyssa finds an empty transmat booth. The capsule, it appears, has been on Earth for the past six years. The Doctor wants to find out more about what the capsule has been doing on Earth, but he finds that the TARDIS cannot leave until the signal has been terminated. Through the Black Guardian's instruction, Turlough boards the ship via the transmat capsule and manages to ingratiate himself with the Doctor. While Nyssa and Tegan wait in the TARDIS, Turlough accompanies the Doctor back to the Earth's surface in the transmat...where he intends to murder him.

The Doctor succeeds in terminating the signal, but instead of returning to 1983, the TARDIS continues back to 1977, from whence the signal originated. Not realizing what has happened, Nyssa and Tegan find a wounded man in the transmat capsule, assume him to be the Doctor, and return him to the TARDIS. While Nyssa watches over him, Tegan goes for help - and eventually finds Brigadier Lethbridge-Stewart, now working as a maths professor at the school. The Doctor, in 1983, also runs across the Brigadier, and soon learns what has transpired six years earlier. But what he does not realize is that the wounded creature, Mawdryn, has his own plans involving the Time Lord. With the Black Guardian and Turlough hunting him on one side, and Mawdryn and his undead brethren on the other, can the Doctor survive with his regenerations intact?